Detecting the interruption of a scanning beam of light by an object can be used to provide information about the location of the object. Specifically, the orientation of the light beam at the time of an interruption event gives the orientation of the object measured from an origin defined by an effective source of the light beam. If two scanning light beams from effective sources whose positions are known are interrupted by an object, the position of the object can be determined by triangulation from the respective orientations of the two beams at the times of the respective interruption events, provided that the position of the object is not colinear with a straight line extending through the effective sources of the two beams.
A coordinate-data input device employing two rotational scanned light beams for determining digital data which encodes the coordinates of the position of an object in a work area swept by the scanned beams is disclosed in U.S. Pat. No. 4,642,422 to Garwin and Levine. In the coordinate-data input device of the patent, each of the two scanning light beams was produced by a rotating beam-scan mirror reflecting a fixed light. The axis of rotation of the two beam-scan mirror constituted an effective source position of the scanning light beam. The two rotating mirrors were positioned near adjacent corners of the generally rectangular work area. A straight line extending between the axes of rotation of the two beam-scan mirrors defined a measurement base line of the device. When an object such as a stylus or human finger was positioned in the work area, the scanning light beams from the rotating beam-scan mirrors intersected the object, which caused light-variation events which were detected by the device. The time of a light-variation event could be used to determine an apparent angle of rotation a beam-scan mirror which defined an intersection angle between the measurement base line and the light beam intersecting with the object. Such rotation angles from the two beam-scan mirrors together with the distance between the axes of rotation of the two beam-scan mirrors could be used in a trigonometric calculation to provide coordinate values for the location of the object in the work area.
A calibration procedure was used in the coordinate-data input device of the '422 patent to correct for certain systematic errors--referred to as angle "index errors"--corresponding to non zero angles between the light beam and the measurement base line at a scan-start time which a control system of the device took as the time the light beam coincided with the base line. The calibration procedure involved positioning three calibration-targets at known relative positions in the work area of the device. According to the '422 patent, the three calibration targets could be positioned in a colinear arrangement, but need not be. A method for detecting and correcting an angle index error associated with either scanning light beam was disclosed in the patent which involved using the event times at which each scanning light beam intersects the three calibration targets, the time interval for the beam to complete one full revolution, and the relative positions of the three calibration targets.
Although the calibration procedure of the '422 is effective to determine and correct angle index errors in rotationally scanned beam-interruption coordinate data-input devices which arise from many sources, the procedure assumes that the rotational velocity of each rotating beam-scan mirror is constant over a full revolution of the mirror, which may not always be the case in certain coordinate-data input devices. Variations in the rotational velocity of a beam-scan mirror which are periodic over each revolution can be one source of angle index errors in a rotationally scanned beam-interruptions coordinate-data input device. Such periodic variations in rotational velocity can also render the calibration procedure of the '422 patent not fully effective to determine and correct such errors.